Particle source



Sept 1952 J. H. COLEMAN 2,611,878

PARTICLE SOURCE Filed Aug. 9, 1950 v a 1 er 2355mm ."jbhi'fbnzm'on I /WZnQma,

' ATTORNEY necessity for eontinuous pumping.

Patented Sept. 23, 1952 JohnIHJCoIeman; Princeton,.N. J1, assignor to Radio Corporation of America, a corporation of Delaware Application August 9, 1950, SerialNo. 178,531

s Claims, 1

The present invention relates to chargedor' neutral particle sources.-

(oi. s s-es) In conventional icnsources, the particles are ionized by electron bombardment in a space which is only partially evacuated. lhe ions are then drawn by electricfields, as by an ion gun, through a communicating aperture into another space external to thefirst, in which second space they are to-be analyzed. 1

These systems have certain disadvantages among which may be mentioned the fact that continuous pumping; is usually necessary to prevent the pressure from rising too high and that ,the equipment is generally bulky and diflicult of construction. Furthermore, it is particularly-inconvenient to ionize-solid state materials.

It is an object of the present invention topro vide an improved chargedparticlesource.

It is a further object of the invention to provide a source of ionsfwhichis moreflexible in use and which readily produces ions-of solid state substances. V I

A further object of the invention is toproduce an ion sourcein a high vacuum without the Another object of the invention is to produce positive or uncharged particles at the-election of anoperator, for analysis-in a mass spectrograph or other instrument.-- i

These and other objects, advantages, features of the invention will bemore apparent from the following description when taken in connection with theaccompanying drawingin which the sole figure represents an ion source which is suited for use in conjunction with'amass spectrograph. V I

In acoordancewith. the inventionand to provide means for analyzingsolid specimens, there is provided means first for producing ions iirom the residual gas in-the system by bombardment by trapped electrons. Thence.thes'e gas ions are caused to bombard the. solid. specimen, thereby releasing other ions and neutral particles therefrom. The neutralparticles (that is uncharged molecules or fragments thereof) may be subjected totrapped electron bombardment andare ionized. All of the ions thence are projected into a conventional ion analyzing space. The electron trapping structures for producing ions aredisclosed in applicants copending U. S. application, Serial No. 93,324, filed May 14, 1949; ,Briefiy, in such structures, electrons are drawn by cold emission from one of two electrodes and biased tosuitable potentials bounding atrapping space subjected to an axial magnetic field. Residual and novel trapping space.

' electron trapping space 22.

ping space22 is substantially toroidal.

2 gas in the space is ionized, and some of the ions thence, are swept out of the trapping space by suitably applied negative accelerating potentials.

In a preferred embodiment of the invention,

the first trapping space is an annular region surrounding an axially disposed solid specimen. Ions from the specimen thence are projected axially through a second trapping space where the neutral particles from the specimen are ionized, and finally all of the ions are further '-projected axially and accelerated into the ion analyzing region of the spectrometer. 1

Referring now more particularly to the sole figure, a cylindrical electrode It isconnectedas an anode to a 3+ supply.

A cathode electrode is connected on a lower potential point on the 3-}- supply and comprises a cylindrically wound grip l2 and a portion Mb of top plate l i'and bottom end plates Hand -6 (at the top and bottom respectively as viewed. in the figure). A central electrode I8 is connected to a conventional ground, to which the negative terminal of the B+ supply is COIIHGCtEQMThG electrode [8 has on itssurface a sample 20to be analyzed in a mass spectrograph orthe like; The cathode, including portion Ma of the upper end plate! and the lower plate 16 together with the I apertured electrode l2 and the anode H} define an The electron trap- The grid I2; together with a portion Ma of the upper end plate l land with a grid structure 24, defines an auxiliary space 26 surrounded by the electron trapping space. The grid 24 has connected 'to it acentral electrode 28 which is surrounded by cylindrical electrode 30 connected at some suit able point on the B-lsupply lower inpotential than the connection of the cathode 12. The electrodes V245 and 28 are grounded. Electrodes 24 and 28 together with cylindrical electrode 3.0 de-,

fine a space 34 which may be a second electron This second space has a 'lower electrode 32, a portion 32a of which is grid-like in construction as shown. A cylindrical space 36 is defined by the electrode 32,]2. cylindrical electrode 38, and an apertured electrode 40. Electrodes t2 and 44 are also apertured or have slits.

A magnetic field is provided as schematically in} dicated by the magnetic pole pieces designated N and S. All of the electrodes thus far described are enclosed Within an envelope 46, and the pole pieces N, S, or other means for producing a magnetic field may be located outside the envelope if desired. ,The'envelope is preferably highly evacu- 1 ated and sealed.

In operation, the electrode 1 acts as an anode and the other electrode comprising electrodes [2, Mb and [6 together acts as a cathode concave to the anode and surrounded by it. The space 22 sodefined is thus an electron trapping space. With a suitable magnetic field applied, the trapped electrons circulate. .circumferentially within this space. Eventhough ithe envelope 45 is highly evacuated, there remain occasional gas particles in the electron trappingspace. These particles are eventually struck by one or more circulating electrons, and the gas particles are ionized, producing positive and negative particles. The negative particles are trapped-in the electron trapping space, as explained in my co-pending application, Serial No. 93,324, filed May 14, 1949, entitled Improvement in GaslRectifieriTube Employing Magnetic Field. In fact, the form .of the electron trapping space and the electrode structure may conform to that taught in my said co-pending application, except that an auxiliary space such as '26 surrounds or is surrounded "by g the electron trapping space and by the cathode,

and except that the two spaces communicate throughone' ormore apertures or openings in the cathode electrode, in this instance through the openings between the grid wires o'i' grid 12.

Other co-pending applications of mine which disclose trapping spaces and their utilization are application SerialNo. $50,530, filed March '18, 1950, entitled Discharge Tube Voltage Transformer; application Serial No. 67,796, filed December 29, 1948, entitled Improvement in Direct Current Transformer; and application Serial No. 57,463, filed October '30, 1948, entitled Electrical Generators.

These positive particles or positive ions created in the trapping region :22 are drawn by the electrio field toward the grid I2. Most of them pass through the grid openings through which the electron trapping space 2 2 and the auxiliary space 26 communicate. Grid 2 is at a higher potential than electrode .18 which has the coating of the sample 2-0. Accordingly, the positively charged particles find themselves in a hold that impels them toward the sample 23. The sample 20 is therefore bombarded by the positively charged particles. The bombardment causes the release from the solid sample '20 of three types of particles-positively charged, negatively charged, or neutral particles. Of course, the order of the electrodes inside to outside, could be reversed, with the positive ion permeable cathode intermediate the anode and cathode. However, the present arrangement is preferred because the ions from the sample are thus readily concentrated in the line of the axis.

Firstdirecting attention to the case in which it is desired to analyze the positively charged particles or ions. These positively charged particles are drawn toward and through grid 24 into the secondary space 34. Thence by suitable relative potentials on grid 32a and electrodes 28, 40, 42, and 44, the openings in the latter electrodes may be made to act as a lens system in which the positively charged particles are focused by the fields resulting'from' the increasingly negative potentials to pass out of the aperture in the last one 44 and into a mass spectrograph.

Second, directing attention to the case in which it is desired to analyze the neutral particles from sample 2!). The neutral particles from space 26 enter the secondary trapping space 34 by diffusion, as electrode 30 is made sufiiciently positive with respect to grid 24 and electrode 28 so that the space 34 is an electron trapping space, in accordance with the principles hereinbefore mentioned. At least some'of these difiused neutralparticles are struck by the trapped electrons in space 34, resulting in ionization and secondary positive particles. These secondary positive particles are drawn into the space 36 by suitable po- '.tential (negative with respect to electrode 28) applied to the grid 32a and may be focused by a conventional electrode system suitable for the purpose. The electrode focusing system illusttrated includes electrodes 38, 40, 42, and 44. Electrode 38 is a cylindrical focusingring, Electrodesflfl, 4'2, 'and 14 may be the customary slit- "type focusing electrodes. By analysis of the positive :partic'les drawn from space 34 arising from bombardment of :the neutral particles diffused from space 26 into space 34, the neutral particles from the sample 20 may be indirectly analyzed.

It will be apparent that there is disclosed herein an improved and superior means for producing charged or neutral particles. The said means comprise an-electron'trapping space and an auxi-liary spacesurrounded'by the'trapping space and having openings communicating with said trapping space. Either positivelycharged or neutral particles from the sample-bombarded by the positively charged particles drawn through the communicating openings may be selected for analysis in a secondary space. If neutral particles are to be analyzed, the secondary space by application of suitable potentials to the various electrode may be a second electron trapping space, at the election "of "the operator causing the potentials to be applied by switches or the like. The flexibility of operation is highly useful, since it saves construction of an alternative arrangement, and particles from samples, such as the sample 20, are readily and easily analyzed. Furthermore, continuous pumping of the envelope 4B is not required. "The envelope 46 may be highly evacuated. Even with extremely high evacuation and extremely low pressure, the circulating electrons in the trapping space 2 2 produce a plentiful supply of charged particles by electron collision with stray free gas particles. These collisions are plentiful because of the number of times the electrons circulate in the trapping space and notwithstanding the long mean free path between gas molecules.

It will be apparent that I have disclosed a novel, improved charged or neutral particle source, flexible in use, and which does not require continuous pumping.

What is claimed is:

1. In a charged particle source, the combination comprising an anode and a cathode surrounded thereby and concave thereto, said anode and cathode defining therebetween an electron trapping space, said cathode defining and surrounding an auxiliary space and having one or more openings through which said two spaces directly communicate, and a central electrode in said auxiliary space and a coating thereon of a sample to be analyzed.

2. In a charged'particle source, the combination comprising a first electrode, an anode elec trode, and a cathode electrode, substantially coaxially arranged, said first electrode and said cathode electrode defining therebetween a first space and said cathode and anode electrodes defining therebetween a second trapping space, said cathode electrode having openings through which said spaces directly communicate, said cathode electrode being positioned intermediate said first electrode and said anode electrode, and a further group of electrodes defining a secondary space situated axially spaced from said first space. said group of electrodes having an opening through which said first and secondary spaces communicate.

3. In a charged particle source, the combination comprising an anode and a cathode surrounded thereby and concave thereto, said anode and cathode defining therebetween an electron trapping space, said cathode defining and surrounding an auxiliary space and having one or more openings through which said two spaces directly communicate, and a further group of electrodes defining a secondary space situated axially spaced from said auxiliary space, said group of electrodes having an opening through which said auxiliary and said secondary spaces communicate, said group of electrodes comprising an electrode of separation through which said secondary and auxiliary spaces directly communicate, a substantially cylindrical anode, an electrode central to said secondary space and to said cylindrical anode, and another electrode which with said electrode of separation and said central 1 electrode define a concave cathode surrounded by said cylindrical anode, said group of electrodes defining a toroidal secondary electron trapping space.

4. In a charged particle source, the combination comprising an anode and a cathode surrounded thereby and concave thereto, said anode and cathode defining therebetween an electron trapping space, said cathode defining and surrounding an auxiliary space and having one or more openings through which said two spaces directly communicate, said electron trapping space being substantially toroidal, a secondary group of electrodes defining a secondary electron trapping space also substantially toroidal and having an electrode of separation between said auxiliary space and said secondary electron trapping space through which said latter two spaces communicate.

5. The combination claimed in claim 4, further comprising a group of focusing electrodes for focusing positively charged particles originating in said secondary electron trapping space.

6. In a charged particle source, the combination comprising a first electrode, an anode electrode, and a cath de electrod su ta ially ooaxially arranged, said first electrode and said cathode electrode defining therebetween a first space and said cathode and anode electrodes defining therebetween a second electron trapping space, said cathode electrode having openings through which said spaces directly communicate, said cathode electrode being positioned intermediate said first electrode and said anode electrode, and a group of further electrodes defining a secondary electron trapping space situated axially spaced from said first space, said group of further electrodes having an opening through which said first and secondary spaces communicate.

7. The combination claimed in claim 6, said group of electrodes comprising an electrode of separation through which said first and secondary spaces directly communicate a substan-- tially cylindrical second anode, a second cathode concave to and substantially coaxial with said second anode and including said electrode of separation one of said second cathode and second anode being surrounded by the other.

8. The combination claimed in claim 6, said further group of electrodes comprising electrodes arranged to focus positively charged particles originating in said secondary electron trapping space.

9. The combination claimed in claim 6, further comprising means to provide a magnetic field having magnetic vectors substantially axially directed throughout both said trapping spaces.

10. The combination claimed in claim 1, further comprising means to produce a magnetic field having magnetic vectors substantially axially directed throughout said trapping space.

JOHN H. COLEMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,767,218 Kunsman June 24, 1930 1,931,254 Doering Oct. 17, 1933 2,272,374 Kallmann et a1 Feb. 10, 1942 2,409,038 Hansell Oct. 8, 1946 2,416,289 Fisk Feb. 25, 1947 2,49 ,2 9 Backus Fe 2 19 0 

